To drive an AC servo motor or a stepping motor, it is necessary to control an electric current passing through the stator of the motor in accordance with the rotational position of a predetermined magnetic pole of the rotor (hereinafter referred to as "the magnetic pole position") relative to the stator of the motor. In this connection, a detector is required, which is capable of detecting the magnetic pole position of the rotor to generate a signal representative of the detected magnetic pole position of the rotor. Conventionally, the detection of magnetic pole position of the rotor has been carried out by:
(1) using a synchro or resolver which has poles equal in number to that of the rotor; PA1 (2) using a detector, which is constructed to detect the absolute position of the rotor at intervals of a period equal to a value calculated by dividing a rotational period of a motor by a number of poles of the rotor or half the number of the rotor poles so that a detected absolute data indicates the magnetic pole position of the rotor; and PA1 (3) using a position detector which is coupled to the rotor of a motor by way of gears or the like so as to rotate a number of revolutions equal to a number of rotor poles or half the number of the rotor poles per one revolution of the motor to output an absolute detection data in one revolution of the detector as a rotor magnetic pole position data.
However, none of the above-mentioned conventional methods can carry out absolute detection for one revolution of a motor, so that they require another detector to effect this kind of absolute detection. Moreover, to detect a rotor magnetic pole position, these methods use a detector constructed in conformity with a number of rotor poles. That is, they have to use different detectors for motors which have a different number of poles, resulting in the necessity of preparing many kinds of detectors for the magnetic pole position detection. Accordingly, problems such as increased complexity in both designing and manufacturing occur. Furthermore, in the above-mentioned method (3) using gears, abration of the gears and backlash can occur, which adversely affects detection accuracy.